Motorola Micor tone decode and squelch discussion, by Karl Shoemaker, AK2O
"PL" is a Motorola trade mark meaning "private line" which has nothing to do with operational status of a system, being open or closed. PL does not mean "please leave". The private line name came about possibly to make commercial customers think they had a private channel and could not hear anyone else on "their" channel. In reality, F.C.C. requirement to monitor the channel before transmitter sometimes didn't work out very well. Motorola had DPL as well, for digital PL. For this discussion we will stay with TPL (tone PL). Other manufactures have the equivalent such as "Channel Guard" (GE) and "Quiet Channel" (RCA). The universal acronym is CTCSS which stands for Continuous Tone Coded Squelch System. For amateur systems that "PL" slang became another name for CTCSS.
Some explanation on how this works may be in order. For this discussion we'll call the CTCSS tone, just "tone". The user's station needs to transmit their carrier along with a continuous sub-audible sinusoidal, single tone in order to activate the system's input. The tone frequencies usually are around from 67 to 200 Hz, although recent standards have increased the range significantly. Lower frequencies are less distracting, however, higher frequencies having slightly faster access, to avoid cutting off the first word of a user's transmission. With some user discipline, any tone frequency will work. Most current radios have this feature while most older ones can be upgraded.
Amateur arrangements and issues
This document is based on R and D, construction, installation and operation of the "Omak Rx" package in 1999. Remote receivers for SRG are set up for tone operation, except for the "Spokane" receivers. One set of issues appeared with using "PL", or CTCSS (tone) receivers. When the stock micor PL deck is used the only modification on the desk is removing a couple pins to disable the high-pass audio filter. On the micor audio board there are several modifications. One major reason is the type of squelch used. The PL deck's output, called PL indicate (PLI) is an active going high. This goes into IC/U 202 logic telling the squelch to stay open during a valid tone decode, regardless of the squelch (knob) setting. Thus, the name "OR" came from squelch open (with PL) or just PL present.
That means a long burst will be heard for non-reverse burst users, because the PL reed will vibrate a little while after the user signal is gone. We don't want this. To correct this problem the audio board is changed to an "AND" squelch, meaning, both the tone and squelch needs to be active for an open audio path. Another way to see this is an "AND" is a variable sensitivity squelch. (vary with the knob on the panel). There were a few reasons for this at the time of early design. Some of them are:
However, for co-channel usage this can be bad also. Here's two points:
In other words, if another repeater (system) was on the same frequency on a certain tone, and a user from the other system (and other tone) starts transmitting, it "disables" the first repeater system. Anyone attempting to get into the first system won't (from the second other user). To make matters worse the first user would be unaware this is happening, causing user confusion and frustration. Tone access has to be properly thought out before using. Cooperation with fellow Amateur's is needed.
It should be remembered after 2005 a real (signal-to-noise) voter replaced the TDV to provide good switching for the Spokane coverage area. As time permitted the receivers squelch's were turned down to the (out lying standard) sensitivity of the 5 dbq point. The philosophy behind this to the fact it's better to get a noisy signal though the system instead of none. Obviously, the (Spokane area) voter will select the best (quietest) user signal.
The Micor squelch-Introduction
When a user signal stops transmitting there is a time period where a laud burst of noise is heard during squelch closure. This typically less than a second for conventional receivers in the industry, however is still annoying to hear. The Motorola Micor radio receiver has a special squelch. It operates in two time constant modes, long and short. From threshold setting to around 20 dbq (db quieting) the circuit is in the long mode. Anything quieter than that the circuit is in the short mode. This is the best-world compromise between practical range of a system and user friendliness. The idea in the commercial world is a weak, noisy, moving mobile will still be heard in the long mode, therefore the long burst is tolerable. For strong, quiet signals the short mode provides a nice, (click) sound for squelch closure. Commercial systems normally use PL "OR" squelch in the base station to ensure the audio path is kept open during traffic. Amateur systems normally are on carrier squelch, therefore, at the mercy of the time constant of the squelch. The same goes for Amateur systems on tone, using the "AND" squelch arrangement. Others would call an AND squelch a variable sensitivity adjustment for that squelch to operate.
A problem - in detail
A problem was discovered around 1999 on the 147.20 repeater.
While an Ellensburg mobile was talking, Spokane reception would hear occasional drop-outs.
This would occur when the (Ellensburg) mobile was around 35 db of quieting while higher speed motion, say above 30 MPH, with muti-path,
sometimes when a quick swish occurs. (which causes the "cos" to blank out just for a fraction of a second).
For the local area coverage this was not a problem (Ellensburg to Ellensburg) however, with the additional (scanning) links the drop-out time was
At this point it's a good idea to point out the pin-out assignments for the squelch IC are difference between the mobile and base stations.
Tests could not be done at the site of the affected receiver at the time of discovery and thought on the problem. Therefore a (spare) mobile receiver was used in the lab, so the "mobile" description-designator (described above) will be discussed here. When observing pin 10 of IC202 which is the "receiver un squelch indicator" (for the mobile board) this blank can be seen with an oscilloscope with a very slow DC trace. This point of IC202 in turn drives the two shunts, pins 6 and 7. Pin 7 is used for the cos output (with the 10K pull up) for SRG equipment. (It was also observed that using a much lower value for the pull up causes the speaker audio to be lower. Reason unknown at this time, but is believed to be irrelevant to the issue being discussed).
It was concluded the best possible reason for the blanking at pin 10 was the receiver's squelch was in the short mode. To "slow" this circuit down C229 on pin 13 was increased from the .22 to 1 uf. Later at the remote site the 1 uf was just tack soldered on the PCB run, leaving the (stock) cap in place, so effectively, C235 would now be a 1.22 uf cap. (remember, it's "C235" in the compa-base version). This is the case in the "Wenatchee Rx" with the TLN6006 series audio and squelch board (non-unified chassis compa station). This caused the "short burst" to be a little longer for stability, but still pleasant to listen to. Rather than a little "click", a larger, fatter "click" would be heard on squelch closure. The "long squelch burst" is still functional, which switches in when the signal is around 20 db quieting or less. Additional research after 2009 will plan to change (not add) the value of C229/C235 to 1.00 uf using a tantalum type capacitor.
Other squelch notes
Analysis of the frequency response of the noise amplifiers would be in order as time permits, to see if the original blanking problem was upper harmonics fooling the squelch, from the quick multi-path fade. Incidentally, this fade is very difficult to duplicate on the bench. Having a buddy in the mobile out there helps with this test. There are scientific procedures to duplicate this in the lab, probably using a room with moving antennas either physically or electrically switching with metal reflectors to cause multi-path. Multi-path is common in the Pacific Northwest with mountains in many repeater service areas; one main reason for having a repeater in the first place !
The condition was "duplicated" by the Author in the shop/lab. With the receiver under test, sitting on the bench a signal generator (with sufficient) level was connected to a piece of small coax, with a short (rubber duck) antenna terminated on the far end. With a medium signal quieting the receiver (not hard-limited) the antenna was swirled around quickly, in a circular motion. While observing the carrier indicator pin on an oscilloscope the DC squelch gate was observed to intermittently change state only for a brief moment (fraction of a second), therefore the swirling of the antenna simulated multi-path in the lab environment.
During the R'nD, another interesting side effect (good or bad) was noticed. You might want to be aware of it. Originally, with a stock arrangement, with the squelch control at threshold, noisy signals have the long burst and stronger (more quieting) signals have the short one. If you turn up the squelch control (increase the noise gain to the noise amplifiers) this raises the noise reference so even more quieter signals (than earlier) still have a long or longer bust than before. Example, if the squelch control is at maximum a signal around 25 db quieting still has a long burst. This makes sense since you are increasing the noise to the noise amplifiers, which "looks" like a noisier signal to them. Now, with the modification of changing C229/C235 almost the reverse happens, when the squelch is at max the burst tends to be on the shorter side. The very short (drop out issue) should not happen as mentioned before, because of increased constant. With the squelch back to threshold it tends to be longer burst. Not a problem, just observations with the particular receiver under test.
For the squelch IC, CAI on Pin 13 can drive a high impedance input or anything higher that 100K. If lower, it will load and affect the IC's squelch operation, disabling the short burst and making all squelch closes a very long burst, which is undesirable. If desired, this pin 13 can drive a high impedance buffer, and used to drive a cor/audio board. Since this is an analog DC output you would need to set a second "DC squelch" adjustment for desired signals. The bias, or reference on the op-amp would work nicely for this method. This method was investigated and decided not to be used for this receiver package. The shunt (pin 7) with resistor pull up was used (in the case of the mobile board).